Exhaust emission control device

ABSTRACT

Fuel added to exhaust gas is uniformly afforded to catalytic surfaces of a NO x -adsorption reduction catalyst so that overall regeneration of the NO x -adsorption reduction catalyst proceeds efficiently. 
     The invention is directed to an exhaust emission control device with the NO x -adsorption reduction catalyst incorporated in an exhaust pipe (exhaust flow passage) for guidance of the exhaust gas  9  from an engine  1  and with an fuel addition device (fuel addition means)  13  arranged for addition of fuel as a reducing agent to the exhaust pipe (exhaust flow passage)  11  upstream of the reduction catalyst  12  (exhaust flow passage)  11 , a dispersion plate  15  being arranged between a position of adding the fuel by the fuel addition device  13  and the NO x -adsorption reduction catalyst  12  for dispersing the exhaust gas  9  to stimulate dispersion of the added fuel.

TECHNICAL FIELD

The present invention relates to an exhaust emission control device.

BACKGROUND ART

Conventionally, exhaust emission control has been attempted using anexhaust emission control catalyst incorporated in an exhaust pipe. Knownas such exhaust emission control catalyst is a NO_(x)-adsorptionreduction catalyst having a feature of oxidizing NO_(x) in exhaust gasfor temporary adsorption in the form of nitrate salt when air/fuel ratioof exhaust is lean, and decomposing and discharging NO_(x) throughintervention of unburned HC, CO and the like for reduction andpurification when O₂ concentration in the exhaust gas is lowered.

Since no further NO_(x) can be adsorbed once an adsorbed NO_(x) amountincreases into saturation in the NO_(x)-adsorption reduction catalyst,it is periodically required to lower the O₂ concentration in the exhaustgas flowing into the reduction catalyst to decompose and dischargeNO_(x).

For example, in application to a gasoline engine, lowering theoperational air/fuel ratio in the engine (operating the engine with richair/fuel ratio) can lower the O₂ concentration and increase thereduction components such as unburned HC and CO in the exhaust gas forfacilitation of decomposition and discharge of NO_(x). However, in useof a NO_(x)-adsorption reduction catalyst in an exhaust emission controldevice for a diesel engine, it is difficult to operate the engine withrich air/fuel ratio.

Thus, it has been necessary that, while an operation with low λ (λ: airexcessive ratio) through fuel injection control is conducted as much aspossible at engine side, fuel (HC) is added to the exhaust gas upstreamof a NO_(x)-adsorption reduction catalyst, the added fuel being reactedas reducing agent with O₂ on the reduction catalyst so as to lower theO₂ concentration in the exhaust gas (see, for example, Reference 1).

[Reference 1] JP 2001-73748A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

However, in such fuel (HC) addition upstream of the NO_(x)-adsorptionreduction catalyst, mist in the fuel added to the exhaust gas is led tothe reduction catalyst without dispersed satisfactorily and tends tononuniformly hit on catalytic surfaces, so that there may occur asituation that the regenerative reaction locally proceeds only aroundcatalytic surfaces on which the mist of the fuel has been attached, andoverall regeneration of the catalyst does not effectively proceed,disadvantageously resulting in lowering of recovery ratio ofNO_(x)-adsorption sites occupied in volume of the reduction catalyst andfailing in educing maximal NO_(x) reduction performance in theNO_(x)-adsorption reduction catalyst

The invention was made in view of the above and has its object touniformly afford fuel added in exhaust gas to catalytic surfaces of aNO_(x)-adsorption reduction catalyst so that overall regeneration of thereduction catalyst may proceed efficiently.

Means or Measures for Solving the Problems

The invention is characterized in that a NO_(x)-adsorption reductioncatalyst is incorporated in an exhaust flow passage for guiding exhaustgas from an engine, fuel addition means for addition of fuel as reducingagent being provided in the exhaust flow passage upstream of thereduction catalyst, a dispersion plate being provided between a positionof adding fuel by the fuel addition means and the reduction catalyst soas to disperse the exhaust gas to stimulate dispersion of the addedfuel.

The dispersion plate may be composed of, for example, at least either ofpunching metal and metal mesh.

Thus, the exhaust gas added with fuel by the fuel addition means passesthrough the dispersion plate to be dispersed, which stimulates thedispersion of the added fuel to equalize mist dispersion in the exhaustflow passage. As a result, uniform fuel supply to the catalytic surfacesof the NO_(x)-adsorption reduction catalyst is realized so that overallregeneration of the reduction catalyst proceeds efficiently.

Dispersion of the mist in the fuel added to the exhaust gas through itspassing through the dispersion plate stimulates refinement orhyperfination of the mist in the fuel. This action of stimulating thehyperfination of the mist also enhances reactivity on the catalyticsurfaces of the NO_(x)-adsorption reduction catalyst in comparison withbefore to attain improvement of regeneration efficiency.

Preferably, in the invention, a catalytic regenerative particulatefilter is arranged just behind the NO_(x)-adsorption reduction catalyst,which makes it possible to capture the particulates entrained in theexhaust gas having passed through the reduction catalyst to attainconcurrent reduction in amount of NO_(x) and the particulates.

The unifomization in reaction of the added fuel in the front-endNO_(x)-adsorption reduction catalyst brings about elevation intemperature of the overall back-end particulate filter with nolocalization through resultant reaction heat, so that the particulatescaptured are satisfactorily burned off to attain early regeneration ofthe particulate filter.

Excessive fuel for use to regeneration of the front-endNO_(x)-adsorption reduction catalyst is oxidized in the back-endparticulate filter, so that HC finally remaining in the exhaust gas anddischarged out of a vehicle is substantially reduced.

EFFECTS OF THE INVENTION

According to the above exhaust emission control device of the invention,the following excellent features and advantages can be obtained.

(I) The fuel added to the exhaust gas can be satisfactorily dispersed,which can bring about the equalization in mist dispersion in the addedfuel and the refinement or hyperfination of the mist, so that highlyreactive hyperfine fuel mist is uniformly afforded to the catalyticsurfaces of the NO_(x)-adsorption reduction catalyst. As a result,effective regeneration of the NO_(x)-adsorption reduction catalyst canbe realized to educe the maximal NO_(x) reduction performance of theNO_(x)-adsorption reduction catalyst.(II) When a catalytic regenerative particulate filter is arranged justbehind the NO_(x)-adsorption reduction catalyst, the particulatesentrained in the exhaust gas having passed through the NO_(x)-adsorptionreduction catalyst can be captured to realize concurrent reduction inamount of NO_(x) and the particulates. The particulates captured can beefficiently burned off using the heat from equalized reaction in thefront-end NO_(x)-adsorption reduction catalyst, and excessivecombustible components for use to the regeneration of the front-endNO_(x)-adsorption reduction catalyst can be oxidized to substantiallyreduce HC remaining finally in the exhaust gas and discharged to outsideof a vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an embodiment of the invention;

FIG. 2 is a graph showing differences between the NO_(x) reduction ratiowhether the dispersion plate exists or not in FIG. 1; and

FIG. 3 is a graph showing differences in regenerative treatment timewhether the dispersion plate exists in FIG. 1 or not.

EXPLANATION OF THE REFERENCE NUMERALS

-   1 engine-   9 exhaust gas-   11 exhaust pipe (exhaust flow passage)-   12 NO_(x)-adsorption reduction catalyst-   13 fuel addition device (fuel addition means)-   14 casing (exhaust flow passage)-   15 dispersion plate-   16 particulate filter

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the invention will be described in conjunction with thedrawings.

FIG. 1 shows an embodiment of the invention. In FIG. 1, referencenumeral 1 denotes a diesel engine, the engine 1 comprising aturbocharger 2 with a compressor 2 a and a turbine 2 b. Intake air 4from an air cleaner 3 is fed via an intake pipe 5 to the compressor 2 aof the turbocharger 2, the intake air 4 pressurized being fed to andcooled by an intercooler 6, the cooled intake air 4 from the intercooler6 being further fed to an intake manifold 7 and dispersed intorespective cylinders 8 in the engine 1.

Exhaust gas 9 discharged from the respective cylinders 8 of the engine 1is fed via an exhaust manifold 10 to the turbine 2 b of the turbocharger2, the exhaust gas having driven the turbine 2 b being guided to theexhaust pipe (exhaust flow passage) 11 and passing through aNO_(x)-adsorption reduction catalyst 12 incorporated in the exhaust pipe11, the gas with reduced NO_(x) there being discharged outside of avehicle.

Arranged in the exhaust pipe 11 and at an entry side of theNO_(x)-adsorption reduction catalyst 12 is a fuel addition device 13 asfuel addition means for injection of the fuel into the exhaust pipe 11,the fuel added by the fuel addition device 13 being utilized as reducingagent for regeneration of the NO_(x)-adsorption reduction catalyst 12.

Arranged in the exhaust pipe 11 and in an entry portion of a casing(exhaust flow passage) 14 carrying the NO_(x)-adsorption reductioncatalyst 12 is a dispersion plate 15 which disperses the exhaust gas 9to stimulate dispersion of the added fuel, the dispersion plate 15 beinga ventilation structure composed of, for example, punching metal and/ormetal mesh.

In the embodiment shown, arranged in the casing 14 and just behind theNO_(x)-adsorption reduction catalyst 12 is a catalytic regenerativeparticulate filter 16 carrying an oxidation catalyst so as to capturethe particles entrained in the exhaust gas having passed through thefront-end reduction catalyst 12.

In the figure, reference numeral 17 denotes an EGR pipe; 18, an EGRvalve; and 19, an EGR cooler.

Thus, with the exhaust emission control device as constructed in theabove, the exhaust gas 9 added with the fuel by the fuel addition device13 passes through the dispersion plate 15 to be dispersed to therebystimulate dispersion of the added fuel into uniformization of the mistdistribution in the casing (exhaust flow passage) 14. As a result,uniform fuel supply to the catalytic surfaces of the NO_(x)-adsorptionreduction catalyst 12 is realized so that the overall regeneration ofthe NO_(x)-adsorption reduction catalyst 12 proceeds efficiently.Dispersion of the mist of the fuel through its passing through thedispersion plate 15 stimulates refinement or hyperfination of the mistin the fuel, so that, through such action of stimulating thehyperfination of the mist, the reactivity on the catalytic surfaces ofthe NO_(x)-adsorption reduction catalyst 12 is enhanced in comparisonwith before to improve the regeneration efficiency.

Especially in the embodiment, the catalytic regenerative particulatefilter 16 is arranged just behind the NO_(x)-adsorption reductioncatalyst 12, so that the particles entrained in the exhaust gas 9 havingpassed through the front-end NO_(x)-adsorption reduction catalyst 12 canbe captured by the back-end particulate filter 16 to attain concurrentreduction in amount of NO_(x) and the particulates.

Moreover, the uniformization in reaction of the added fuel in thefront-end NO_(x)-adsorption reduction catalyst 12 brings about elevationin temperature of the overall back-end particulate filter 16 withoutlocalization through the resultant reaction heat, so that theparticulates captured are satisfactorily burned off to attain earlyregeneration of the particulate filter 16.

Thus, according to the embodiment, the fuel added to the exhaust gas 9can be satisfactorily dispersed to attain unifomization of mistdispersion in the added fuel and hyperfination of the mist, so that thehighly reactive hyperfine fuel mist can be uniformly afforded to thecatalytic surfaces of the NO_(x)-adsorption reduction catalyst 12 torealize the effective regeneration of the reduction catalyst 12 andeduce maximal NO_(x) reduction performance of the reduction catalyst 12.

In fact, it has been ascertained by validation experiments conducted bythe inventors that, as shown in the graph of FIG. 2, the NO_(x)reduction ratio is enhanced in a case where a dispersion plate 15 isarranged in comparison with a case no dispersion plate 15 is arranged.

Especially in the embodiment, the particles entrained in the exhaust gas9 having passed through the NO_(x)-adsorption reduction catalyst 12 canbe captured to realize concurrent reduction in amount of NO_(x) and theparticulates and, moreover, the particulates captured can be efficientlyburned off using the heat from the uniformized reaction on the front-endreduction catalyst 12.

FIG. 3 shows experimental results in validation on regenerativetreatment time to the particulate filters 16 (treatment time necessaryfor forced burning-off of the captured particulates through positiveaddition of fuel); it has been ascertained that the regenerativetreatment time can be substantially reduced in a case where thedispersion plate 15 is provided than in a case where no dispersion plate15 is provided.

When the catalytic regenerative particulate filter 16 is provided inback-end, any excessive combustibles in use to regeneration of thefront-end NO_(x)-adsorption reduction catalyst 12 can be oxidized tosubstantially reduce HC remaining finally in the exhaust gas 9 anddischarged outside of a vehicle.

It is to be understood that an exhaust emission control device accordingto the invention is not limited to the above embodiment and that variouschanges and modifications may be made without leaving the spirit of theinvention. For example, the dispersion plate may be composed of anymaterial other than punching metal and metal mesh.

1. An exhaust emission control device wherein a NO_(x)-adsorptionreduction catalyst is incorporated in an exhaust flow passage forguiding exhaust gas from an engine, fuel addition means for addition offuel as reducing agent being provided in the exhaust flow passageupstream of the reduction catalyst, a dispersion plate being providedbetween a position of adding fuel by the fuel addition means and thereduction catalyst so as to disperse the exhaust gas to stimulatedispersion of the added fuel.
 2. An exhaust emission control deviceaccording to claim 1, wherein the dispersion plate is composed of atleast either of punching metal and metal mesh.
 3. An exhaust emissioncontrol device according to claim 1, wherein a catalytic regenerativeparticulate filter is arranged just behind the NO_(x)-adsorptionreduction catalyst.
 4. An exhaust emission control device according toclaim 2, a catalytic regenerative particulate filter is arranged justbehind the NO_(x)-adsorption reduction catalyst.